Why Is the Letter Z Associated with Sleeping?

Z isn’t associated with sleeping, specifically, but rather with snoring. ... Z as shorthand for snoring is a relatively recent invention. It came into common use with the advent of comics. ... the first use of “z-z-z” to represent snoring given in the OED is from a 1924 publication by the American Dialect Society, implying it was in popular use some time before. straightdope.